Downloading the game is easy, and within around 10 minutes you'll be on the track racing. Firstly just make sure you have the following software downloaded, these NEED to be downloaded in order for you to play the game... so if you are having problems opening and signing into the game, this may be your problem: 

.Net FrameWork 2.0 -

http://www.microsoft.com/downloads/details.aspx?FamilyID=0856EACB-4362-4B0D-8EDD-AAB15C5E04F5&displaylang=en

The Latest version of Direct X -

https://www.microsoft.com/downloads/details.aspx?FamilyID=2da43d38-db71-4c1b-bc6a-9b6652cd92a3&displaylang=en

Once you have both of those downloaded, you're ready! go to either

http://www.download.com/Street-Challenge-Extreme-Velocity/3000-7520_4-10772032.html?part=dl-StreetCha&subj=dl&tag=button

or

http://www.StreetChallenge3D.com/signup.aspx?referal=nd4spd41